DE102017120619B4 - Extension device for a bone screw and screwdriver instrument - Google Patents

Abstract

The invention relates to an extension device (1) for a bone screw (4) and to a screwdriver instrument (2) connectable to the extension device (1); It is proposed that the screwdriver instrument (2) comprises a toggle mechanism (62) with a first and a second lever arm (64, 66), which are pivotally connected to each other via a common hinge point (68), that the first lever arm (64) Area of its free end (70) at a first pivot bearing point (72) on the extension device (1) is releasably attachable and pivotally mounted in the attached state and axially supported, and that the second lever arm (66) at a second pivot bearing point (74) the shaft is pivotally mounted and thereby axially supported, that the first lever arm (64) has an elastic elongation and that this Längendehnbarkeit and a distance of the second pivot bearing point (74) of the common hinge point (68) of the lever arms (64,66) to each other are tuned that when manually pivoting the second lever arm (66) in the direction of a parallel alignment with the axial Längsrich tion (16) a dead center is overcome and the toggle mechanism (62) in a tilt-stable position (60) passes, in which the shank of the screwdriver instrument (2) in the axial direction (16) is forced against the Werkzeugansetzstelle the bone screw.

Description

The invention relates to an extension device for a bone screw, in particular for a pedicle screw, in particular in minimally invasive spinal surgery, and connectable to the extension device screwdriver instrument having an axial longitudinal direction, a concentric thereto circumferential direction and a radial direction, wherein the extension device extends in the axial longitudinal direction is and forms an access channel to the bone screw and releasably attachable to the bone screw, and wherein the screwdriver instrument has a shaft which can be arranged in the axial longitudinal direction within the access channel and rotationally fixed to a tool attachment point of the bone screw can be coupled.

Extending devices in question here can comprise, for example, a sleeve-shaped housing part which can be detachably connected to a head of the bone screw and delimits a preferably tunnel-shaped access channel to the bone screw. Through this access channel, the surgeon can access the bone screw and, in particular, couple a screwdriver instrument with a tool attachment site of the bone screw. But there are also extension devices conceivable, which are formed from at least one shell part.

WO 2017/089141 A1 shows a screwdriver instrument which is releasably attachable to a bone screw as a whole. It has a tubular or sleeve-shaped housing part and an internal rod part which can engage in a torque-receiving tool attachment point of the bone screw. Between the tubular or sleeve-shaped housing part and the inner rod part a toggle mechanism is provided. After screwing in the bone screw, the screwdriver instrument is loosened again as a whole from the bone screw.

US 2013/0110179 A1 shows an instrument for producing a permanent connection between a screwed into the bone bone screw and inserted into the bone screw correction rod. The instrument comprises an outer sleeve member which engages an outer locking sleeve on the bone screw, and a lever mechanism in which one of the legs bears against a rod disposed within the instrument rod member which at the other end presses the correction rod against the bone screw from above. Thus, the locking sleeve is forced upwards on the bone screw and thus prevents the correction rod from escaping lateral encompassing legs of the bone screw to the outside, so that the correction rod is held on the bone screw without requiring a grub screw. The instrument thus serves to produce this assembly connection between the bone screw and the correction rod.

The present invention has for its object to provide a screwdriver instrument for a bone screw or an aggregate or a system comprising an extension device and a screwdriver instrument for a bone screw, the screwdriver instrument can be reliably connected to the extension device and a tool attachment point of the bone screw and should also be felt by a surgeon as easy and appropriate to use.

This object is achieved by an extension device and a screwdriver with the features of claim 1.

The inventive design of the extension device and the screwdriver instrument with a toggle mechanism, the instrument can be brought by a single manual adjustment movement in the form of pivoting of the second lever arm in a correct operating position on the bone screw and on the releasably connected to the bone screw extension device. Due to the axial force, which is applied by the toggle mechanism and with which the screwdriver instrument is forced in the axial direction against the Werkzeugansetzstelle the bone screw, the instrument is rotationally fixed and reliably held due to the bias voltage in its correct mounting and operating position on the extension device. This opens up the possibility that the surgeon can effectively handle the extension device and the bone screw as intended by holding and operating the screwdriver instrument.

According to a preferred embodiment of the invention, the toggle mechanism is formed so that the first lever arm has lateral bearing cheeks, with which it can laterally surround the extension device, and that the first lever arm with these lateral bearing cheeks forms the first pivot bearing point to the extension device. Here, the lever arm engages with its lateral bearing cheeks the extension device, which opens the possibility to pivotally couple the lever arm with the extension device.

For this it proves to be advantageous that are formed in the lateral bearing cheeks of the first lever arm in the direction of the extension device projecting bearing journals. These journals can be releasably or non-detachably connected to the bearing cheeks, or they can only pass through the bearing cheeks pivotally.

The elongation of the first lever arm can be realized in various ways. For example, elastically extensible materials could be used for this, or the first lever arm could be formed with an elastically yielding region. According to a further preferred embodiment of the toggle mechanism is formed so that the first lever arm is formed with an elastic Längendehnbarkeit in which it has a substantially in the axial longitudinal direction extending longitudinal portion, but also extends slightly arcuate, thereby resulting in the elongation. This slightly arcuate course of the longitudinal section allows in a simple manner, a Längendehnbarkeit of the first lever arm, which further allows that the toggle mechanism can get over its dead center in a tilt-stable position.

It may also prove advantageous if the first lever arm has a longitudinal section extending substantially in the axial longitudinal direction and at one or both ends a section extending transversely or obliquely to the axial longitudinal direction. This also makes it possible to realize a type of bow or trapezoidal shape which can lead to a longitudinal extensibility of the first lever arm. In particular, an additional elongation in length can be realized thereby, if in some other way, ie in particular by a slight arc shape of the longitudinal section is given a Längendehnbarkeit.

It is further conceivable that the above-mentioned to the axial longitudinal direction transversely or obliquely extending portion of the first lever arm forms a bearing cheek for the lever arm.

It may also prove to be advantageous and advantageous if the second lever arm has a bearing cheek, wherein the joint joint with the first lever arm and the second pivot bearing position is formed with the shaft, the joint joint and the second pivot bearing in the closed state of the toggle mechanism approximately on a line in the axial longitudinal direction.

To form the second pivot bearing point of the second lever arm is pivotally connected in any manner with the shaft of the screwdriver instrument releasably or permanently coupled. For this purpose, in turn, bearing pins, pins or the like pivotable coupling elements between the components can be used. It can also be provided in an advantageous manner that the second pivot bearing point comprises a particular detachable from the shaft component with a through hole which can be pushed onto the shaft in the axial longitudinal direction and rotatably or rotatably and axially supported at a predetermined or predeterminable position on the shaft is fastened, and that the second lever arm is pivotally connected to this component. In this way, the mentioned releasable component can simplify the assembly of the components.

The lever arms could be formed from essentially flat material sections, possibly with bearing cheeks extending therefrom. It also proves to be advantageous if the first and / or the second lever arm are rounded in the form of a surface in the sense that they partially surround the two shell parts from radially outside. As a result, the feel or ergonomics of the screwdriver instrument can be optimized.
Furthermore, it proves to be advantageous if the second lever arm has a manually graspable area, which is formed with respect to the second pivot bearing point on the side remote from the common pivot point half of the second lever arm and radially outward. When attaching the instrument to the extension device then only one of the closing hand following setting pressure on the instrument or on the second lever arm needs to be exercised.

Furthermore, it proves to be advantageous if the second lever arm has at its free end an obliquely to the axial longitudinal direction radially outwardly projecting end portion which is tangible from radially inward to the lever arm from its closed parallel to the axial longitudinal direction aligned closed position into a release position to move over the dead center of the toggle mechanism.
It can prove to be advantageous that the extension device comprises a sleeve-shaped housing part, which limits the access channel to the bone screw, and that the first pivot bearing point is formed on the sleeve-shaped housing part.

Alternatively, it may prove advantageous that the extension device comprises at least one shell part which is releasably, but rigidly fixed in the longitudinal direction and also rotatably on the head of the bone anchor, and that the first pivot bearing point is formed on the shell part.

In a further development of this idea, it can prove to be advantageous for the extension device to comprise two shell parts which are preferably separate and elongated in the longitudinal direction and together define the access channel to the bone screw in the releasably fixed state, and in that the first pivot bearing point on one of the shell parts or is preferably formed in the circumferential direction between the shell parts, that is formed by two shell parts.

In such case, it proves to be advantageous if the first lever arm of the screwdriver instrument is thereby mounted on both shell parts, the first lever arm engages with protruding bearing pin in a respective shell part or engages a respective shell part and is pivotally supported there and in the axial longitudinal direction.

In a further development of this idea, it proves to be advantageous that the first pivot bearing point is bounded by flanks of the respective shell part, which in the proximal direction is preferably rounded toward one another. In this way, the screwdriver instrument can be guided in its intended connection and support position on the two shell parts. As a result, a certain degree of self-centering is thus achieved when attaching the instrument to the shell parts.

It may also prove advantageous that the flanks of the respective shell part define an engagement opening in the shell part. This engagement opening is preferably also extends in the circumferential direction. It thus offers the possibility that the first lever arm is moved transversely to the axial longitudinal direction into the respective engagement opening and then moved in the proximal direction into the pivot bearing position.

Further details, features and advantages of the invention will become apparent from the appended claims and from the drawings and the following description of a preferred embodiment of the extension device according to the invention with screwdriver instrument and bone screw.

• 1a-f verschiedene Ansichten einer an einer Knochenschraube lösbar festgelegten Verlängerungsvorrichtung und eines an der Verlängerungsvorrichtung lösbar festgelegten Schraubendreherinstruments;
• 2a-f verschiedene Ansichten des Schraubendreherinstruments nach Figuren la-f;
• 3a-d verschiedene Ansichten einer an einer Knochenschraube lösbar festgelegten Verlängerungsvorrichtung und eines an der Verlängerungsvorrichtung lösbar festgelegten Schraubendreherinstruments gemäß einer weiteren Ausführungsform;
• 4a-d verschiedene Ansichten der Knochenschraube und der Verlängerungsvorrichtung.

In the drawings show:

• 1a-f various views of a releasably fixed to a bone screw extension device and a releasably fixed to the extension device screwdriver instrument;
• 2a-f different views of the screwdriver instrument according to figures la-f;
• 3a-d various views of a releasably fixed to a bone screw extension device and a releasably fixed to the extension device screwdriver instrument according to another embodiment;
• 4a-d different views of the bone screw and the extension device.

The 1a-f show an embodiment of an extension device according to the invention 1 and a screwdriver instrument 2 , The extension device 1 is in a releasable retention connection to a bone anchor 4 represented in the form of a pedicle screw shown by way of example.

How out 4 ad obviously the bone anchor comprises 4 a screw shaft 6 which is polyaxially pivotable with respect to a U-shaped head when viewed from the side 8th the pedicle screw 4 is arranged. At the head 8th is an example of a so-called "long head", with longer thighs 10 is formed, wherein a predetermined breaking point 12 is provided, on which the long head can be shortened in the implanted state by a head part 13 is separable. Nevertheless, the head includes 8th of the bone anchor 4 in known manner a tangible area 14 in the form of a grippable groove on the outer circumference of the head 8th is trained. The tangible area 14 of the head 8th can otherwise according to the disclosure in the patent application DE 10 2014 225 327A1 the applicant whose content is to be included in the disclosure of the present application.

The extension device 1 includes an axial longitudinal direction 16 , a concentric circumferential direction 18 and a radial direction 20 who are equally upside down 8th of the bone anchor 4 Respectively. The extension device 1 in the case illustrated by way of example comprises two separate and longitudinal ones 16 extended shell parts 22 , which are exemplary and preferably formed corresponding to each other. The shell parts 22 are substantially rigid and torsionally stable formed in the sense that they are almost completely dimensionally stable, but this does not exclude a slight irrelevant deformability, especially due to the length. The shell parts 22 are detachable on the head in a manner to be described 8th of the bone anchor 4 fastened and then form a limited between them working channel 24 through which the surgeon with other instruments access to the bone anchor 4 and the operating environment can take.

A respective shell part 22 is essentially dimensionally stable, it comprises in exemplified case, however, a first spring element 28 which is in the axial longitudinal direction 16 is extended and transverse to this approximately radially outwardly deflectable. The spring element 28 is in the illustrated embodiment, for example, in one piece with the shell part 22 trained and goes to his proximal end 30 in one piece in a wall 32 of the shell part 22 over. The spring element 28 is through a material-free slot 34 in the wall 32 of the shell part 22 limited. The material-free slot 34 runs at its proximal ends curved, resulting in a uniform voltage introduction into the shell part 22 causes. The spring element 28 further includes a radially inwardly projecting support area 36 on, by means of which the spring element 28 and thus the shell part 22 in the axial longitudinal direction 16 against the head 8th of the bone anchor 4 is supportable. This support area 36 is in the longitudinal direction 16 for example, proximal to a radial projection 38 of the shell part 22 arranged. The radial projection 38 is in the circumferential direction 18 in the form of an inwardly projecting rib, which also has an extension in the proximal direction, in order to engage behind the area 14 to be able to intervene at the head of the bone anchor. One recognizes best in 4d the extension of the radial projection 38 in the circumferential direction 18 , The radial projection 38 indicates in the embodiment shown here by way of example 4a-d a central break 40 on, so that the spring element 28 in the longitudinal direction 16 distally through the radial projection 38 can extend through. It could also be in front of the radial projection 38 end up. In the area distal to the radial projection 38 includes the spring element 28 in the case exemplified a radially inwardly projecting engagement region 42 which is in an intervention recess 44 at the head 8th of the bone anchor 4 can engage and an additional positive connection between the shell part 22 and the bone anchor 4 can produce. The spring element 28 is further designed to be in the area of the central break 40 of the radial projection 38 a rejuvenation or constriction 46 identifies, so there is narrower in the circumferential direction. As described above, the relevant shell part 22 detachable but with respect to all degrees of freedom form-fitting on the head 8th of the bone anchor 4 kept that as best as possible 4 ad it can be seen with its radial projection 38 the reachable area 14 of the head 8th of the bone anchor in the longitudinal direction 16 and in the radial direction 20 back-hooked and in the opposite direction with its support area 36 axially against the head 8th supported. In the above-described embodiment, the spring element engages 28 continue with its distal engagement area 42 into the intervention recess 44 at the head 8th on. Out 3b It can be seen that the respective shell part 22 two inwardly drawn longitudinal edge areas 54 has, with which it with lateral support surfaces 55 from in the circumferential direction 18 opposite sides against the thigh 10 of the head 4 of the bone anchor abuts. The shell part 22 so embraces the thigh 10 essentially at least almost free of play and is thus also in the circumferential direction 18 kept non-rotatable in addition to the action of the engaging elements.

Overall, a form-locking operating connection with respect to all degrees of freedom is thereby created, which can be achieved by the spring element 28 of the respective shell part 22 in the radial direction 20 is deflected to the outside. This can be achieved by accessing the spring element by means of a release instrument (not shown) 28 is taken.

At the of the two shell parts 22 formed extension device 1 is the screwdriver instrument 2 detachably arranged.

The screwdriver instrument 2 includes an elongated shaft 60 and one pivotally with the shaft 60 connected toggle mechanism 62 , The toggle mechanism 62 includes a first lever arm 64 and a second lever arm 66 that communicate with each other via a joint joint 68 pivotally connected. The first lever arm 64 is in the realm of his free end 70 at a first pivot bearing point 72 on the extension device 1 pivotally and detachably attachable and in the axial longitudinal direction 16 supportable. The second lever arm 66 is at a second pivot bearing point 74 on the shaft 60 pivotally mounted while also supported axially. The first lever arm 64 has an elastic Längendehnbarkeit, wherein this elongation and a distance of the second pivot bearing point 74 from the joint joint 68 the two lever arms 64 . 66 are matched to one another such that during manual pivoting of the second lever arm 66 in the direction of the arrow 76 (see. 1e) a dead center of the toggle mechanism 62 is overcome and the toggle mechanism arrives in a tilt-stable position in which the shaft 60 of the screwdriver instrument 2 in the axial direction 16 against the tool attachment site of the bone anchor 4 is forced.

The 2a to f show the components of the screwdriver instrument 2 , One recognizes 2 B that a distal end 78 of the shaft 60 as a rotary coupling means 80 is trained. It can be complemented in the tool attachment site of the bone anchor 4 to achieve a rotary coupling.

The first lever arm 64 includes a longitudinal section 81 and at its free end 70 lateral bearing cheeks 82 with which he made the two shell parts 22 laterally overcame. With these lateral bearing cheeks 82 are in the direction of the extension device 1 protruding bearing journals 84 intended. With these side bearing cheeks 82 and journal 84 engages the first lever arm 64 in a respective engagement opening 86 (S. 1d) on. This intervention opening 86 is from flanks 88 limited in the proximal direction 90 Rounded to run towards each other. In this way, the first pivot bearing position 72 formed by the lateral bearing cheeks 82 with their journals 84 transverse to the axial longitudinal direction (arrow 92 ) in the respective engagement opening 86 the shell parts 22 swung in and then in the proximal direction 90 against the mutually running flanks 88 be moved and finally against it are axially supported. To form the joint joint 68 includes the first lever arm 64 at its further free end 94 also lateral bearing cheeks 96 , where also here journal 98 are provided, which with lateral bearing cheeks 100 at the second lever arm 66 are pivotally coupled.

The second pivot bearing 74 between the second lever arm 66 and the shaft 60 is exemplified by one of the shaft 60 detachable component 102 with a passage opening 104 on the shaft 60 is pushed on and means in a transverse opening 106 of the shaft 60 engaging journal 108 on the shaft 60 is determinable. In the example shown, these journals pass through 108 radially outward the second lever arm 66 in the area of its lateral bearing cheeks 100 , The bearing cheeks 100 are there with a through hole 110 educated.

If now starting from the in 1e illustrated initial situation by manual pressure on a manually reachable area 112 is exercised, so can the second lever arm 66 in the direction of the arrow 76 pivot, which causes the first lever arm 64 axially at the first pivot bearing point 72 supported and in the axial longitudinal direction 16 slightly widens in the longitudinal direction, so that a dead center of the toggle mechanism 62 can be overcome. As a result, an axial setting pressure within the shaft 60 towards the tool attachment site of the bone anchor 4 transfer. The components of the toggle mechanism are matched to one another such that the toggle mechanism 62 After overcoming a dead center in a tilt-stable position is located. In this in 1f shown position form the extension device 1 and the screwdriver instrument 2 one rigid with each other and with the bone anchor 4 connected unit.

For loosening the screwdriver instrument 2 indicates the first lever arm 64 at its free end obliquely to the axial longitudinal direction 16 outwardly projecting section 114 on, which is tangible from within, around the lever arm 66 again in a release position, so contrary to the arrow 76 to be able to move.

The screwdriver instrument 2 can be particularly simple and stable on the already connected to the bone anchor extension device 1 arrange and solve it again. This is the shank 60 of the screwdriver instrument 2 in the access channel 24 inserted until its Drehkopplungsmittel 80 into the tool attachment point of the bone anchor 4 engages, then then the first lever arm 64 transverse to the longitudinal direction in the engagement opening 86 swung in and then by closing the second lever arm 66 in the direction of the arrow 76 against the first pivot bearing 72 drawn.
The 3a to d show a further embodiment, which differs from the above-described in that the extension device 1 not just the two shell parts 22 includes, but an additional sleeve-shaped housing part 120 is provided, which on the two shell parts 22 in the axial longitudinal direction 16 is deferrable. In the pushed state is the sleeve-shaped housing part 120 with one or both shell parts 22 latched so that it is captive, but releasably connected to the shell parts. How to get out 3a until c recognizes, is the sleeve-shaped housing part with a protruding recess 122 in the area of the intervention opening 86 the shell parts 22 educated. Thus, the screwdriver instrument 2 in the same way as previously described on the shell parts 22 to be ordered.

It would also be conceivable in an embodiment not shown that instead of the two shell parts 22 a single, for example, sleeve-shaped housing part at the head of the bone anchor 4 is detachably arranged and the screwdriver instrument is then arranged on a first pivot bearing point on this single sleeve-shaped housing part.

Claims (18)

Extension device (1) for a bone screw (4), in particular for a pedicle screw, in particular in minimally invasive spinal surgery, and with the extension device (1) connectable screwdriver instrument (2), with an axial longitudinal direction (16), a concentric thereto circumferential direction (18) and a radial direction (20), wherein the extension device (1) extends in the axial longitudinal direction (16) and an access channel (24) to the bone screw (4) and releasably attachable to the bone screw, and wherein the screwdriver instrument (2) has a shank (60) which can be arranged in the axial longitudinal direction (16) within the access channel (24) and rotationally fixed with a tool attachment point of the bone screw ( 4), the screwdriver instrument (2) comprising a toggle mechanism (62) having first and second lever arms (64, 66) pivotally connected to each other via a common pivot (68), the first lever arm (64) in the region of its free end (70) at a first pivot bearing point (72) on the extension device (1) is detachably attachable and pivotally mounted in the attached state and axially supported, and wherein the second lever arm (66) at a second pivot bearing point (74) is pivotally mounted on the shaft and thereby axially supported, wherein the first lever arm (64) has an elastic elongation and this Län Elongation and a distance of the second pivot bearing point (74) of the common pivot point (68) of the lever arms (64,66) are coordinated such that during manual pivoting of the second lever arm (66) in the direction of parallel alignment with the axial longitudinal direction (16) a dead center is overcome and the toggle mechanism (62) in a tilt-stable position (60) passes, in which the shank of the screwdriver instrument (2) in the axial direction (16) is forced against the Werkzeugansetzstelle the bone screw. Extension device and screwdriver instrument after Claim 1 , characterized in that the first lever arm (64) has lateral bearing cheeks (82) with which it can laterally surround the extension device (1), and in that the first lever arm (64) engages with these lateral bearing cheeks (82) the first pivot bearing point (72 ) to the extension device (1). Extension device and screwdriver instrument after Claim 2 , characterized in that at the lateral bearing cheeks (82) of the first lever arm (64) in the direction of the extension device projecting bearing pin (84) are formed. Extending device and screwdriver instrument according to one of the preceding claims, characterized in that the first lever arm (64) is formed with an elastic Längendehnbarkeit in which it has a substantially in the axial longitudinal direction (16) extending longitudinal portion (81), but also easy arcuate runs. An extension device and screwdriver instrument according to any one of the preceding claims, characterized in that the first lever arm (64) has a longitudinal section (81) substantially in the axial longitudinal direction and at one or both ends (70, 94) an axial longitudinal direction (16). have transversely or obliquely extending portion. Extension device and screwdriver instrument after Claim 5 , characterized in that the axial longitudinal direction transversely or obliquely extending portion of the first lever arm (64) forms a bearing cheek (82). Extending device and screwdriver instrument according to one of the preceding claims, characterized in that the second lever arm (66) has a bearing cheek (100), wherein the common hinge point (68) with the first lever arm (64) and the second pivot bearing point (74) with the Shaft (60) is formed, wherein the common hinge point (68) and the second pivot bearing point (74) in the closed state of the toggle mechanism (62) lie approximately on a line in the axial longitudinal direction (16). Extending device and screwdriver instrument according to one of the preceding claims, characterized in that the second pivot bearing (74) comprises a of the shaft (60) in particular releasable component (102) having a through hole (104) in the axial longitudinal direction (16) on the Shaft (60) can be pushed and fastened at a predetermined or predeterminable position on the shaft rotationally fixed or rotatable and axially supported, and that the second lever arm (66) on this component (102) is pivotally articulated. Extension device and screwdriver instrument according to one of the preceding claims, characterized in that the first and / or the second lever arm (64,66) are formed rounded in the sense that they surround the two shell parts (22) partially from the outside radially. Extending device and screwdriver instrument according to one of the preceding claims, characterized in that the second lever arm (66) has a manually engageable region (112) with respect to the second pivot bearing point (74) facing away from the common hinge point (68) half of the second lever arm (66) and is formed radially on the outside. Extension device and screwdriver instrument according to one of the preceding claims, characterized in that the second lever arm (66) at its free end (94) has an obliquely to the axial longitudinal direction radially outward projecting end portion (144) which is tangible from radially inward to move the lever arm (66) from its closed parallel to the axial longitudinal direction (16) aligned closed position into a release position. Extending device and screwdriver instrument according to one of the preceding claims, characterized in that the extension device (1) comprises a sleeve-shaped housing part (120) which limits the access channel (24) to the bone screw (4), and in that the first pivot bearing point (72) on the sleeve-shaped housing part (120) is formed. Extending device and screwdriver instrument according to one of the preceding claims, characterized in that the extension device (1) comprises at least one shell part (22) which is detachable but rigid in the longitudinal direction (16) and also rotationally fixed to the head (8) of the bone anchor (4). is fixable, and that the first pivot bearing (72) on the shell part (22) is formed. Extension device and screwdriver instrument after Claim 13 , characterized in that the extension device (2) comprises two preferably separate, and in the longitudinal direction (16) extending shell parts (22) in the releasably fixed state on the bone screw (4) together limit the access channel (24) to the bone screw and that the first pivot bearing (24) is preferably formed on both shell parts (22). Extension device and screwdriver instrument according to one of the preceding claims, characterized in that the first lever arm (64) with projecting bearing pin (84) in the extension device (1), in particular in a respective shell part (22) engages, or on the extension device, in particular a respective shell part, engages and is pivotally supported there and in the axial longitudinal direction (16). Extension device and screwdriver instrument after Claim 13 . 14 or 15 , characterized in that the first pivot bearing point (72) of flanks (88) of the respective shell part (22) is limited to run in the proximal direction, preferably rounded to each other. Extension device and screwdriver instrument after Claim 16 , characterized in that the flanks (88) of the respective shell part define an engagement opening (86) in the shell part (22). Extending device and screwdriver instrument according to one of the preceding claims, characterized in that the shell part (22) in the releasably secured state by means of a radial projection (38) a hintergreifbaren region (14) on the head (8) of the bone anchor (4) in the longitudinal direction (16 ) and also in the radial direction (20) engages behind.

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